Conventionally, in a plant cultivation field, a producer or the like has visually observed leaves of cultivated plants in order to check health states of the plants, and has removed disordered portions (such as leaves) that are damaged by pests or the like.
In recent years, a cultivation method of cultivating plants in a large scale has become popular, with increase in cultivation area. If the cultivation area increases, a producer or the like has difficulty in managing health states entirely in a cultivated plant cluster in the manner similar to the conventional method (removal of disordered sites by visual observation, for example).
There has been proposed a technique of diagnosing a health state of a plant in terms of a physiological function of the plant, with no visual observation.
For example, there is a technique of managing a health state of a plant by evaluating a photosynthetic function, which is closely relevant to the health state of the plant. In this technique, photochemical reaction is driven by light energy absorbed by chlorophyll, and chlorophyll fluorescence emitted by the chlorophyll is measured to quantitatively evaluate the photosynthetic function in the plant. This technique is referred to as the chlorophyll fluorescence image measurement method because chlorophyll fluorescence is measured by means of an image. In this chlorophyll fluorescence image measurement method, chlorophyll fluorescence emitted by chlorophyll that is important in photosynthesis is measured. It is thus possible to directly diagnose photosynthetic reaction. Accordingly, physiological functional disorder of a plant, in other words, invisible disorder from outer appearance, can be diagnosed.
For example, Patent Documents 1 and 2 each disclose a technique of diagnosing a health state of a plant in accordance with this chlorophyll fluorescence image measurement method.
Patent Document 1 discloses a technique of diagnosing a growth state of a plant by irradiating the plant with laser light and measuring laser excited fluorescence released from a leaf of the plant.
According to Patent Document 1, the health state of the target plant can be diagnosed by measuring chlorophyll fluorescence of a predetermined wavelength released from the leaf having received laser light and comparing a measurement value with a reference value of a healthy plant and a reference value indicating a growth limit of withering or the like. These reference values are set preliminarily.
Patent Document 2 describes diagnosis of a health state of a plant by measuring chlorophyll fluorescence of a predetermined wavelength released from a leaf having received laser light and estimating a chlorophyll content in the target plant from a measurement value in accordance with a calibration curve that is formed preliminarily and indicates the relationship between chlorophyll concentration in a healthy plant and chlorophyll fluorescence.
In the large-scale cultivation method in recent years, plants of a same breed are cultivated in a large scale. If one of the plant individual bodies carries pests, damage rapidly expands to other adjacent plant individual bodies. With slight delay in taking action, there is a risk of expansion of pests entirely in the cultivated plant cluster. If the cultivated plant has pests or the like, fruits obtained from this plant decrease in crop yield and quality in comparison to those obtained from a healthy plant individual body, thereby causing a huge loss to a producer and the like.
In particular, tomato russet mite as one type of rust mites are common pests that appear throughout a year in structured cultivation and wither stems and leaves, while fruits are damaged to have rough skins. Such tomato russet mite have extraordinary high biotic potential and rapidly expand to other adjacent plant individual bodies in a few days. Detection in an early stage is important for inhibition of damage.
At the initial stage of appearance of the pests, leaves and the like do not have disorder such as withering. It is thus impossible to recognize damage from outer appearance. Damage cannot be recognized from outer appearance until a stage of expansion of the damage to some extent, in other words, until tomato russet mite expand from a plant individual body to other adjacent plant individual bodies.
Meanwhile, tomato russet mite are highly sensitive to agricultural chemicals and can be easily removed by spraying a small amount of an agricultural chemical to a plant individual body having initial damage if such a plant individual body having initial damage can be detected. In order to inhibit disorder by tomato russet mite, it is quite important to detect appearance and remove tomato russet mite in an early stage. This applies also to other pests that rapidly expand to other adjacent plant individual bodies in a few days with high biotic potential and are highly sensitive to agricultural chemicals.
Upon such removal, it is possible to apply the technique of Patent Document 1 or 2 for detection of a plant individual body having pests or the like. These techniques enable detection of a withered plant individual body but cannot achieve detection of a plant individual body carrying pests or the like in an initial stage.
Currently, there is developed no diagnostic method that achieves detection of disorder in an initial stage of a plant individual body having pests or the like. Desired therefore is development of a plant health diagnostic method in an early stage, which is applicable to the plant cultivation method with a large cultivation area.